Current airport aviation practices depend on the use of de-icing fluid to remove ice and prevent its future build-up for time periods of 5-10 minutes. Verification that wing and other aerodynamic or control surfaces are ice free is done visually, often under difficult viewing conditions. Occasionally significant ice build-ups are not noticed, with tragic results. Responsibility for detecting such ice rests with the aircraft crew who rely on visual viewing, perhaps supplemented with an ordinary flashlight. Obviously, a need exists for a system which is capable of accurately and easily determining the presence of ice on an aircraft wing.
Metallic surfaces and dielectric surfaces behave differently when illuminated with light, particularly with respect to their polarization properties. One of the strongest differences and most easily observable is the property of metals to reverse the rotational direction of circularly polarized light. For example, the specular reflection of right handed (clockwise looking towards the source) circularly polarized light from a metal surface changes it to left handed (counterclockwise) polarization and vice versa. This effect is used in the construction of optical isolators which permit light to initially pass through the isolator but prevent specularly reflected light from returning through the isolator back to the source. The optical isolator is a circular polarizer that is usually implemented from a linear polarizer and a quarter wave retarder plate that has its fast and slow axes located 45.degree. from the polarization axis of the polarizer. The polarizer must precede the retarder in the light path.
When a metallic surface (or surface painted with a metallic paint), such as the wing of an aircraft, is illuminated with circularly polarized light (which may be generated by passing unpolarized light through a circular polarizer) and the reflected energy viewed through the same circular polarizer, the resulting image is extremely dim since the circular polarizer is performing as an isolator with respect to the specular reflection from the metal surface. Other types of surfaces (birefringent, certain dielectric, matte, etc.) viewed through the same circular polarizer maintain their normal brightness because upon reflection they destroy the circular polarization. If the circular polarizer is flipped (reversed) so that the retarder precedes the polarizer, it no longer acts as an isolator for the illuminating beam and the metallic surface's image will now be viewed of normal (bright) intensity.
Most non-metallic and painted or matte surfaces illuminated with circularly polarized light and viewed through the same circular polarizer will maintain their normal intensity. Such surfaces, as well as a coat of ice on the metal, whether matte white due to a snow covering or crystal clear due to even freezing will destroy the circular polarization of the reflected light and therefore take on the depolarizing property of a matte painted surface with respect to the optical isolator. A transparent dielectric over metal depolarizes circularly polarized light passing through it if it has numerous internal point scatterers or is birefringent. Ice has this characteristic. Thus, circularly polarized light reflected from a painted surface, snow, ice, or even transparent ice over metal will be depolarized and will not be affected by the isolator.
Therefore, the image of a clear metal surface that is ice-free will alternate between dark and bright when alternately viewed through an isolator and non-isolator structure, respectively. Apparatus other than the combination of optical isolators and non-isolators can produce the same effect. Any ice or snow covering the metal surface will cause the image to maintain the same brightness regardless of whether it is viewed through an isolator or non-isolator structure or equivalent structures.